The invention relates to an actuator comprising an electric motor used to maneuver a movable element for closure, shading, solar protection or a screen in a building. The invention is particularly useful when one wishes to measure, on the basis of the behavior of the motor, the actions on a movable element subject to the effects of gravitation, for example the movable element being displaced between a bottommost position and a topmost position caused by rotational movements of the motor.
Some actuators intended to be installed in buildings and intended for maneuvering elements for closure, shading, solar protection or a screen (such as for example roller blinds, doors, gates or shutters) comprise a monophase induction motor (or asynchronous motor) with permanent capacitor. The invention is also applicable to actuators comprising an induction motor of three-phase type, or else to actuators comprising a commutator-type or brushless (BLDC) DC motor.
A reduction gear is associated with the motor in the actuator. This reduction gear is partially irreversible; that is to say it has a different efficiency depending on whether it is the input shaft which drives the output shaft or whether it is the output shaft which drives the input shaft, the efficiency generally being lower when it is the output shaft which drives the input shaft.
These actuators are powered from the AC network, for example 230 V 50 Hz, or from a DC source.
They are fitted with an immobilization brake ensuring the disabling of the actuator when the motor is not powered. The role of the brake is essential, in particular in regard to safety. It must be activated by default, and oppose the driving torque caused by gravity on the movable element, this torque itself being able to vary according to the position of the movable element, in particular if the latter is windable, for example a roller blind.
The brake is often deactivated by the magnetic flux of the stator of the motor when it is an induction motor, but it is known to use other types of brakes: electro-brake, hysteresis brake, centrifugal brake, ramp brake etc. The use of such ramp brakes, also called “differential brakes” or “torque transmission brakes”, is particularly beneficial in replacing electro-brakes, brakes activated by the stator flux or hysteresis brakes. These differential brakes have been especially studied by the Applicant in order to prevent the action of the brake from significantly masking the indirect measurement of the torque produced by the load on the actuator when in fact the torque provided by the motor is measured.
However, whatever type of brake is used, a problem exists resulting from the partial irreversibility of the reduction gear: for a given variation in the load torque at the actuator output level, the variation in motor torque is different depending on whether the actuator is operating in driven-load or in driving-load mode.
Now, as has been noted in the prior art, operation in driven-load or in driving-load mode does not depend simply on the direction of rotation of the motor. In the case of a roller blind or similar, ordered to close from a fully wound position, the motor must first of all produce a motor torque to push the slats as unwinding begins, and then it produces a resistive torque (the motor then operating as generator) when the unwound mass is sufficient for its weight to exceed the action of the mechanical friction. Likewise, in the case of a tilting garage door where it is necessary firstly to push over the mainly horizontal part of the trajectory, and then to hold back over the mainly vertical part of this trajectory.
When using a differential brake the situation is worse still, since the motor always operates as motor, whether the load is driving or driven: how then to identify the state of the load?
Even without a differential brake being used, the identification of a driving-load or driven-load situation poses a problem if one is not dealing with using solely a DC motor with magnets and commutator which allows this identification by simply measuring the direction of the current.
In the case where a reduction gear is present, and the lower its efficiency (for example less than 80%), the losses in the reduction gear suffice to prevent to get such a fine detection: it is in fact necessary that the load is not only driving but moreover that it provides a power greater than the losses in the reduction gear to cause the direction of the current to reverse. This effect is all the more marked the more the reduction gear exhibits an irreversible character, that is to say poor efficiency when it is driven by the load.
When using an electronic commutation motor (ECM), for example a motor of brushless type (BLDC), the losses in the power supply device are also such that it is well beyond the switchover to a driving-load situation that this results in a reverse transfer of power and in the need to dissipate this power at the level of the motor.